Human presence in New Mexico can be traced back 20,000 years. Between about 500 B.C. and 1200 A.D., Mogollon Indians lived in valleys near the New Mexico-Arizona border. Anasazi Indians sometimes were cliff dwellers who lived in enormous apartments that contained as many as 800 rooms. Navaho and Apache tribes arrived from the north around 1500, followed by Utes and Comanches. European exploration was due entirely to the Spanish, at first in search of gold. Coronado explored the region from 1540 to 1542. The first colony was established in 1598. In 1610, its capital was moved to Santa Fe, the oldest seat of government in the United States. Catholic missionaries attempted to forcibly convert the Indians to Christianity, and Spanish civil authorities used them for forced labor. They rebelled in 1680 and expelled the Spanish authorities, but in 1692, Spanish control was re-established. In the early 1800s, a few Americans began to penetrate the territory. The Santa Fe Trail was opened by William Becknell in 1821, to provide a conduit for trade between Santa Fe and Missouri. During the Mexican War, American troops took control of the territory. The Treaty of Guadalupe Hidalgo confirmed American possession of the region. The Territory of New Mexico was organized in 1850. The Gadsden Purchase added more territory, and the final boundaries were settled in 1863. Disputes between cattlemen and others led to the Lincoln County War in the 1870s, which required the territorial government to apply martial law and use troops to restore order. New Mexico became the 47th state in 1912. During World War II, the United States government chose isolated New Mexico as the ideal place to develop the atomic bomb. Research and development facilities were built at Los Alamos, and the first atomic explosion took place in the New Mexico desert at Alamagordo on July 16, 1945.